A distributed charge is an explosive assembly of elongated form and with a velocity of detonation in the direction of its length which is approximately that of seismic waves in the surrounding medium. In the case of land prospecting, this assembly is ordinarily detonated in a water-filled section of a well, and the velocity of the surrounding medium is that of the appropriate type of wave in the rock forming the walls of the well. In a marine environment it is the speed of compressional waves in water.
An article entitled "Broomstick Distributed Charge" by S. T. Martner and Daniel Silverman, Geophysics, Vol XXVII, No. 6, Part II, Pages 1,007-1,015 describes the background and general use of distributed charges in seismic surveying, and the types of charges then available. The greatest success obtained in this kind of charge had been achieved by wrapping a linear explosive cord known as Primacord.RTM. (a trade name of the Ensign Bickford Company) wrapped in a helix around a length of material such as wood, plastic or rubber hose. The explosive material was the Primacord alone, and the choice of size of explosive and size and length of the mandrel was made so that the explosive detonation in an axial direction was about the same speed as the velocity of the generated seismic waves in the adjacent walls of the well. The article (and the associated U.S. Pat. No. 2,609,885 of D. Silverman) also showed that it was possible to place lump explosives, also called concentrated charges, along the helix to increase the pressure waves going into the well.
The type of arrangement of this distributed charge resulted in considerably greater ratio of downward to upward axial impulse than that of concentrated charges of equal mass. However, this charge was more expensive than such concentrated explosives. The fact that the helix was wound around the mandrel meant that in forceful loading of such a charge down a well there was likelihood of part of the helix being scraped away by contact with the walls. Also, it was found extremely difficult to waterproof such a distributed charge to the point that the distributed charge could be left for periods of up to or greater than 24 hours in a well before detonation time. Since seismic surveying practices frequently involve necessity of keeping the charge in a water-filled well longer than this period, there was considerable likelihood of misfire, with its attendant difficulties.
Lang in U.S. Pat. No. 3,238,871 teaches the use of an adjustable delay unit between lump explosive charges. The adjustable time delay element uses one single kind of linear explosive (usually light Primacord.RTM.), but permits the adjustment of the physical lengths of the linear explosives in the delay unit by movement of one of two telescoping parts of the mandrel. These change the position of an explosive bridge so that the total length of linear explosive employed could be timed to correspond to the speed of seismic waves in the adjoining rock formation. The difficulty of this arrangement is that the use of such linear explosive cord lengths of which must naturally be bent back and close to other lengths is likely to result in sympathetic detonation across part of the cord, thus mis-timing the appropriate delay.
Silverman in his U.S. Pat. No. 2,770,312 teaches utilization of two different thick linear explosive materials having differing detonating velocities. One detonates at a rate of less than and the other greater than the speed of seismic waves in the surrounding medium. The low speed explosive (sometime called "sausage" powder) consisted of continuous cartridges of extra dynamite with a predetermined velocity dependent upon composition, encased in a plastic tube. By the proper choice of the lengths of the two parts, one can produce a distributed charge which approximates in propagation velocity the speed in the walls of the well. The "sausage" powder could not be made up in linear cords.
Axelson, et al., in U.S. Pat. No. 3,354,826 teaches a distributed charge for use under water, the explosive line charge being made of an explosive strip folded into a stack to form a part of a multiple explosive charge package. The explosive strip is approximately 20 times as long as it is wide and has a thickness of less than 1/30 of its width. Here again, the difficulty lies in the fact that, if the explosive strip is arranged in such a physical shape that part of it is in proximity to another part, there is great probability of sympathetic detonation across the part of the strip and hence malfunctioning of the timing.
A different arrangement was taught in U.S. Pat. No. 3,211,094 Liddiard. In order to shape the detonation wavefront in an explosive charge, a shaped metal plate was interposed between a booster charge and a working charge. The front phase of this plate is conical in shape, with the center part thicker than the edges. This causes the center part of the booster charge to be accelerated first and the outer parts accelerated later. Since the local velocity of the plate varies inversely with the thickness, the front of the plate is deformed to produce a plane surface striking the working charge simultaneously over the entire area. This of course does not produce a distributed charge to the type mentioned above, with the desirable property of matching the speed of seismic waves in the adjoining medium.
Barns in his U.S. Pat. No. 3,196,974 eliminates the timing fuse feature of the distributive charge. Instead his distributed charges are made up of arbitrarily placed lump charges in the well, each of which has associated with it a seismic detector type initiator. By detonating the top explosive, for example with a cap, a seismic wave starts down the walls of the well. Each time a pressure sensitive switch located along the column is energized, it sets off a blasting cap in a lump charge at that point in the well. Such a distributed charge is dangerous to deploy. It is not frequently used. Phase blasters use a similar idea. These are detonators actuated by pressure developed by arrival of the compressional wave from another charge in the same borehole. Such devices were used for a time but were never really considered safe.
Finally, Marke, et al., have issued U.S. Pat. No. 3,712,220 in which the initiating line of detonating cord is used as in the distributed charge described in the first article and the Silverman Pat. No. 2,609,885. Here, however, extra weight of explosive is involved by using explosive adhesively stuck to the detonating cord or attached by a layer of adhesive. This also results in considerable difficulties when loading the hole.
In the past, it has been found that the performance of the distributed charge was very poor when lump explosives were introduced into the assembly. Results were outstanding using the broomstick charge when the required charge size (on the order of 5 lb.) permitted the use of Primacord.RTM. only. However, once the lump explosives were inserted at intervals along the broomstick charge to achieve additional charge weight, the performance was much less satisfactory. Evidently, poor coupling occurred for reasons that were not altogether understood. There are many areas in the Mid Continent area that require charges larger than 5 lb. to obtain adequate depth penetration. In the Wyoming Rockies area for example, charges of 50 lb. and more are almost a necessity.
In the Gulf Coast, formation velocities are 6,000 ft/sec. and less. Velocities that low cannot be matched using "sausage" powder. This velocity required if broomstick charges were to be used, that the Primacord.RTM. be wrapped at a low angle of pitch. When this was done, these charges often cross detonated. Because of this problem, distributed charges were seldom used in operations in the Gulf Coast area. There is a definite need for distributed charges in that area because of the interest in obtaining high frequencies necessary for stratigraphic trap exploration. Also, it is often necessary to shoot 25 lb. or more of explosive in a charge, in order to obtain sufficient penetration.
In the past, it was necessary when using a distributed charge to load and fire the shot within one or two days. If the charge, for any reason, was left to "sleep" longer than that, it was not likely to be successfully detonated. Present technology of recording multifold data requires that a seismic line be drilled and loaded for a week or longer before moving onto the line with a recording truck and the geophone cables. Consequently, it is now essential that the distributed charge be capable of "sleeping" for a week or longer. In the configuration described below, there is little or no limitation based on time between loading and time of shooting such a distributed charge.